Collagen is one of the most abundant animal proteins in nature. It is present in all types of multicellular animals, including humans, where it is estimated to account for about 30% of the total human body protein. Collagen constitutes the fibrillar component of the soft connective tissues (e.g., skin, ligaments, and tendon) and is the major component of the organic matrix of calcified tissues such as bone and dentine. In addition to its structural significance, collagen plays an important role in development and wound healing, and has been implicated in aging and some disease processes.
There are several genetically distinct types of collagen, which are referred to as types I, II, III, and so forth. Type II collagen is the major collagen of cartilage. It is synthesized by chondrocytes as a procollagen molecule with noncollagenous aminopropeptide and carboxypeptide extensions. These two extensions are removed by specific peptideases before type II collagen is incorporated into fibrils.
Type II collagen can be expressed in two forms by differential splicing of the primary gene transcript. The two mRNAs either include or exclude an exon (exon 2) encoding the major portion of the amino propeptide. The mRNA including exon 2 is referred to as type IIA mRNA; the mRNA excluding exon 2 is referred to as type IIB. The proteins encoded thereby are referred to as type IIA procollagen and type IIB procollagen, respectively. The peptide encoded by exon 2 is referred to as type IIA propeptide. Ryan, M. C. and Sandell, L. J., (1990) J. Biol. Chem. 265:10334. Type IIB mRNA is primarily expressed by chondrocytes, while type IIA is expressed in chondroprogenitor cells. Sandell, L. J., et al., (1991) J. Cell Biol. 114:1307.
Osteoarthritis (OA) is characterized by the destruction of articular cartilage. Despite the ultimate degeneration of articular cartilage associated with OA, a striking phenomenon of OA cartilage is an attempt by chondrocytes to repair their matrix. This is characterized by proliferating chondrocyte clones, the development of chondroosteophytes and the synthesis of collagen.
There is a need for cartilage markers that can provide information on collagen metabolism of diseased cartilage. Such markers are useful in estimating the pathological conditions of diseases such as OA. In this regard Shinmei et al ((1993) Osteoarthritis and Cartilage 1:128) have suggested that since type II collagen is a unique component of cartilage, the carboxy-terminal type II procollagen peptide (pCOL II-C) levels in joint fluids could reflect the synthetic activity of type II chondrocytes in the diseased joint, and thus could be used as a simple marker of type II collagen synthesis in articular cartilage in joint diseases. Unfortunately use of pCOL IIC as a marker for OA associated cartilage synthesis, as suggested by Shinmei et al, ((1993) Osteoarthritis and Cartilage 1:128), is not entirely satisfactory since non-diseased chondrocytes synthesize type II collagen, including type IIB collagen. Although there is evidence for increased synthesis of type II collagen at sites of pathological degradation of cartilage, use of pCOL IIC as a marker does not distinguish whether the increased synthesis is "normal" or associated with a disease state. Use of pCOL IIC also does not distinguish between type IIA or type IIB propeptide, since type IIA and IIB propeptides share the same carboxy-terminal ends.
There is a need for assays which can distinguish type IIA and type IIB procollagens, since only type IIA procollagen is unique to disease states such as osteoarthritis.